Since mobile devices, such as a cell phone and a notebook computer, have come to have higher performance, a demand for higher capacity of a lithium ion secondary battery has become stronger. Conventionally, graphite has been mainly used as a negative electrode material for a lithium ion secondary battery, however, aiming at higher capacity, development of a negative electrode material containing an element, which has high theoretical capacity and ability for absorption and desorption of a lithium ion (hereinafter also referred to as “specific element”, and that containing the specific element is also referred to as “specific element substance”), has become active.
As the specific element, silicon, tin, lead, aluminum, etc. are well known. Among others, silicon and silicon oxide have advantages over other specific element substances, since they have a higher capacity, are inexpensive, and have a better processability, negative electrode materials containing them are especially energetically studied.
Meanwhile, the specific element substances are known to cause remarkable cubical expansion when alloyed by charging. Such cubical expansion micronizes a specific element substance itself, and further destroys the structure of a negative electrode material using the same, leading to a breakage of the electrical conductivity. Therefore it has a drawback in that the capacity decreases significantly over cycles.
To eliminate the drawback, a technique has been proposed, by which a specific element substance is micronized and combined with graphite using a carbonic substance or a resin. With such a composite particle, even if a specific element is micronized by alloying with Li, the electrical conductivity can be secured by graphite or a carbonic substance, and it has been known that the cycle performance can be improved significantly compared to single use of a specific element substance as a negative electrode material.
However, still in some cases, the electrical conductivity in a composite particle is broken due to destruction of the composite particle structure by expansion during alloying with Li, and adequate cycle performance cannot be attained. For the sake of absorption and relaxation of the expansion, studies have been broadly conducted focusing on introduction of voids in a composite particle (see, for example, Japanese Patent No. 3466576, Japanese Patent Application Laid-Open (JP-A) No. 2006-228640, Japanese Patent No. 3995050, and Japanese Patent No. 3987853).